It is often necessary to ascertain the height of a stack of sheet material in a printer, such as an ink jet printer or laser printer, and also in other machines where stacks of sheet material are processed such as photocopiers.
A known approach to sensing the height of a stack of sheet material is to rest an actuator on the uppermost surface of the stack. As sheets are either added or removed from the stack, the actuator rides on the stack surface or the uppermost sheet in the stack. The actuator is connected mechanically to a device that senses when the stack has reached a critical height. This critical height may either be a maximum or minimum. For example, sensing a maximum height would be important in ascertaining when an output tray of a printer is full. Sensing a minimum height is important when ascertaining when a paper tray is close to being empty.
A problem with using a mechanical actuator is that there is physical contact of the paper surface, or other such sheet material. Such contact is particularly disadvantageous in output trays used in ink jet printers, since physical contact may smudge or smear the ink used in ink jet printers. However, even in other printers, physical contact may mar the surface of the sheet material, perhaps when the sheet material has a delicate surface, for instance in high gloss papers.
An object of the present invention is to substantially ameliorate at least some of the problems discussed in relation to the prior art, and it is not necessary that the invention in its broadest aspect should overcome each and every one of the abovementioned problems.